The present invention relates to broadcasting sets comprising a wire-dipole rotary antenna, designed to work with decameter waves.
Sets of this kind exist. Their rotary antenna has a fixed base surmounted by a vertical mast which, in association with horizontal girders, most usually supports two parallel vertical curtains of wire dipoles, separated by one or two vertical curtains of horizontal wires that play the role of wave reflectors. Certain antennas have only one curtain of dipoles associated with one curtain of wires. Each curtain of dipoles, with its reflector, constitutes an antenna proper. In certain applications, a portion of the dipoles of this antenna, according to choice, may or may not be powered, this being achieved by means of switching assemblies. Hereinafter, the complete set with the curtains of dipoles and the reflector curtain or curtains shall be called a rotary antenna whenever it is necessary to avoid confusion with the two antennas proper, Which are also called antenna faces or dipole curtains or high-frequency antennas and low-frequency antennas. The latter two terms express the fact that, of the two curtains of dipoles of a rotary antenna, one is designed to work in high frequencies, for example in the 13/15/17/21/26 MHz bands located between 13,600 and 26,100 MHz while the other is designed to work in the low frequencies, for example in the 6/7/9/11 MHz bands located between 5,950 and 12,050 MHz.
The dipoles of each of the two curtains are generally arranged in rows and columns, symmetrically with respect to the projection of the axis of the mast in the plane of the curtain. As for the power supply to the dipoles, it is provided by means of a feeder that sets up links between a transmission source on the ground and either of the two curtains of wire dipoles. For this purpose, a feeder cable goes out from the supply source, goes through the rotary crown between the fixed base and the mast, gets sub-divided into two cables that go out horizontally along two horizontal girders fixedly joined to the mast and gets sub-divided again to go out vertically and supply or feed the different dipoles. It must be noted that there are known ways of using switches in this feeder line to allow or bar the supply of certain dipoles of the antenna in order to use only one curtain when there are two of them, or to use only all or a part of the dipoles of a curtain in order to cause variations in the elevation angle of the curtain considered.
The rotary antennas presently in operation use only one transmission source at a time. This is especially so in high-powered broadcasting. Now, given the cost of a broadcasting antenna whose height goes up to several tens of meters, it would be useful to be able to use these antennas together with two transmission sources transmitting simultaneously. Indeed, broadcasting stations are now increasingly transmitting in dual frequencies in one and the same direction, i.e., one and the same program is sent out in one and the same direction on two different frequencies, for example, 6 and 7 MHz. For this purpose, there is a known way of using two rotary antennas, each with one transmitter. Sending one and the same program on two frequencies is therefore a costly approach since it requires two rotary antennas and since one rotary antenna is very costly.